Rotary electrical machine

ABSTRACT

A rotary electrical machine including an annular stator having a stator core around which an armature winding is wound, and a rotor arranged on an inner circumference of the stator, a permeance of a q-axis magnetic circuit is made larger than a permeance of a d-axis magnetic circuit. The machine includes a switch for supplying power to a field winding and controller. The controller calculates the duty ratio on the condition that an upper limit of the duty ratio of the switch is a predetermined value and turns on/off the switch based on the calculated ratio. The predetermined value is set to a value larger than the duty ratio corresponding to the field current that gives the maximum reduction amount of the inductance of the field winding with respect to an increasing amount of the field current in a range that the current can take and is less than 100%.

CROSS-REFERENCE TO RELATED APPLICATION

This application is the U.S. bypass application of International Application No. PCT/JP2018/026283 filed on Jul. 12, 2018, which designated the U.S. and claims priority to Japanese Patent Application No. 2017-140832, filed on Jul. 20, 2017, the contents of which are incorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates to a rotary electrical machine.

BACKGROUND

Conventionally, as shown in JP H4-255451 A, for example, a rotary electrical machine including an annular stator having a stator core around which an armature winding is wound, and a rotor arranged on an inner circumference side of the stator is known. The rotor includes a field core and a field winding. The field core has a cylindrical boss part and a plurality of claw-like magnetic pole parts arranged on an outer circumferential side of the boss part and having magnetic poles with alternately different polarities formed in a circumferential direction. The field winding is wound on the outer circumferential side of the boss part and generates a magnetomotive force by supplying a field current.

SUMMARY

In a rotary electrical machine according to a first aspect, the rotary electrical machine includes an annular stator having a stator core around which an armature winding is wound, and a rotor arranged on an inner circumference side of the stator. The rotor includes a field core including a cylindrical boss part and a plurality of claw-like magnetic pole parts arranged on an outer circumferential side of the boss part and having magnetic poles with alternately different polarities formed in a circumferential direction of the rotor, and a field winding that is wound on the outer circumferential side of the boss part and generates a magnetomotive force by supplying a field current. In a case where a magnetic circuit in which a magnetic flux formed by a magnetomotive force of the field winding flows and which passes through a d-axis via the boss part, a pair of the claw-like magnetic pole parts and the stator core is referred to as a d-axis magnetic circuit, and a magnetic circuit which is formed by a current flowing through the armature winding and which passes through a q-axis shifted by 90° in an electric angle from the d-axis is referred to as a q-axis magnetic circuit, a permeance of the q-axis magnetic circuit is made larger than a permeance of the d-axis magnetic circuit.

The first aspect further includes a switch provided so as to supply power from a power source to the field winding by being turned on and stop supplying power from the power source to the field winding by being turned off; and a controller that, in a case where a ratio of an on-time to one switching cycle of the switch is defined as a duty ratio, calculates the duty ratio on the condition that an upper limit of the duty ratio is set as a predetermined value and turns on/off the switch based on the calculated duty ratio, and the predetermined value is set to a value which is larger than the duty ratio corresponding to the field current that gives the maximum reduction amount of the inductance of the field winding with respect to an increasing amount of the field current in a range that the field current can take and which is less than 100%.

BRIEF DESCRIPTION OF THE DRAWINGS

The above-described objects, other objects, features, and advantages of the present disclosure will become more apparent from the following detailed description with reference to the accompanying drawings. In the accompanying drawings:

FIG. 1 is a perspective view of a rotary electrical machine according to a first embodiment;

FIG. 2 is a perspective view of a rotor;

FIG. 3 is a sectional diagram of the rotor;

FIG. 4 is a diagram showing an outline of d, q-axis magnetic circuits;

FIG. 5 is a diagram showing an outline of the d-axis magnetic circuit;

FIG. 6 is a partial sectional diagram of the rotor;

FIG. 7 is a diagram for explaining a surface area of a surface opposite to a stator in the rotor;

FIG. 8 is a diagram showing an outline of a magnet magnetic circuit;

FIG. 9 is a diagram showing an electrical configuration of the rotary electrical machine;

FIG. 10 is a characteristic diagram showing relations of an inductance, a time constant, and a torque with respect to a field current and a duty ratio;

FIG. 11 is a block diagram showing processing performed by a controller;

FIG. 12 is a diagram showing a relation between a d-axis current and a predetermined value;

FIG. 13 is a time chart showing changes in the field current when a rotary electrical machine according to the first embodiment is started;

FIG. 14 is a time chart showing changes in the field current when a rotary electrical machine according to a comparative example is started;

FIG. 15 is a diagram showing an electrical configuration of a rotary electrical machine according to a second embodiment;

FIG. 16 is a sectional diagram of a field core according to another embodiment;

FIG. 17 is a perspective view of a field core according to another embodiment;

FIG. 18 is a perspective view of a field core according to another embodiment;

FIG. 19 is a perspective view of a field core according to another embodiment;

FIG. 20 is a sectional diagram of a field core according to another embodiment;

FIG. 21 is a sectional diagram of a field core according to another embodiment;

FIG. 22 is a perspective view of a field core according to another embodiment;

FIG. 23 is a perspective view of a field core according to another embodiment; and

FIG. 24 is a flowchart showing a processing procedure of a controller according to another embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In recent years, for example, in an on-vehicle rotary electrical machine, a need for high output is increasing. In response to this need, the discloser of the present application has created a configuration described below. A magnetic circuit in which a magnetic flux formed by a magnetomotive force of a field winding flows and which passes through a d-axis via a boss part, a pair of claw-like magnetic pole parts and a stator core is referred to as a d-axis magnetic circuit, and a magnetic circuit which is formed by a current flowing through an armature winding and which passes through a q-axis shifted by 90° in an electric angle from the d-axis is referred to as a q-axis magnetic circuit. In this case, the permeance of the q-axis magnetic circuit is made larger than the permeance of the d-axis magnetic circuit. According to this configuration, a torque of the rotary electrical machine can be substantially improved, and for example, when the rotary electrical machine is used as a power generator, a power generation capacity can be substantially improved.

In a magnetic circuit, the permeance and the inductance of the field winding are inversely proportional to the magnetic resistance of the rotor. With a configuration in which the permeance of the q-axis magnetic circuit is made larger than the permeance of the d-axis magnetic circuit, when the field current flows, at least part of the field core is apt to be magnetically saturated, and the discloser of the present application has found that when the field current becomes large, the phenomenon that the inductance of the field winding sharply decreases occurs. Although the inductance of the field winding is saturated and its value becomes small, a field magnetic flux quantity can be made sufficiently large by supplying a large field current, and the torque of the rotary electrical machine can be substantially improved. However, when the inductance sharply decreases and a level change in which the inductance changes by an order of magnitude occurs, a time constant τ sharply decreases and becomes small accompanying this change. The time constant τ is, for example, represented by Lrt/R in a general electrical circuit including a field winding, where the inductance of the field winding is Lrt and the resistance of the field winding is R. When the time constant becomes small, the ripple of the field current becomes large, and there is a concern that controllability of the field current may be greatly lowered.

Incidentally, when a circuit having a small time constant is controlled, a measure for improving the control amount of the field current by raising a switching frequency of a switch constituting the circuit is conceivable. However, this measure increases a switching loss to increase a heat generation amount, and as a result, a heat radiation measure or the like for suppressing reduction in reliability of the switch is required.

The present disclosure has a main object to provide a rotary electrical machine capable of suppressing reduction in controllability of the field current.

In a rotary electrical machine according to a first aspect, the rotary electrical machine includes an annular stator having a stator core around which an armature winding is wound, and a rotor arranged on an inner circumference side of the stator. The rotor includes a field core including a cylindrical boss part and a plurality of claw-like magnetic pole parts arranged on an outer circumferential side of the boss part and having magnetic poles with alternately different polarities formed in a circumferential direction of the rotor, and a field winding that is wound on the outer circumferential side of the boss part and generates a magnetomotive force by supplying a field current. In a case where a magnetic circuit in which a magnetic flux formed by a magnetomotive force of the field winding flows and which passes through a d-axis via the boss part, a pair of the claw-like magnetic pole parts and the stator core is referred to as a d-axis magnetic circuit, and a magnetic circuit which is formed by a current flowing through the armature winding and which passes through a q-axis shifted by 90° in an electric angle from the d-axis is referred to as a q-axis magnetic circuit, a permeance of the q-axis magnetic circuit is made larger than a permeance of the d-axis magnetic circuit.

The first aspect further includes a switch provided so as to supply power from a power source to the field winding by being turned on and stop supplying power from the power source to the field winding by being turned off; and a controller that, in a case where a ratio of an on-time to one switching cycle of the switch is defined as a duty ratio, calculates the duty ratio on the condition that an upper limit of the duty ratio is set as a predetermined value and turns on/off the switch based on the calculated duty ratio, and the predetermined value is set to a value which is larger than the duty ratio corresponding to the field current that gives the maximum reduction amount of the inductance of the field winding with respect to an increasing amount of the field current in a range that the field current can take and which is less than 100%.

The controller of the first aspect calculates the duty ratio on the condition that an upper limit of the duty ratio is set as the predetermined value and turns on/off the switch based on the calculated duty ratio. The predetermined value is set to a value which is larger than the duty ratio corresponding to the field current that gives the maximum reduction amount of the inductance of the field winding with respect to an increasing amount of the field current in a range that the field current can take and which has a value less than 100%. According to this setting, the field current can be made to flow immediately before the inductance of the field winding saturates, and ripple of the field current can be suppressed. This makes it possible to suppress reduction in controllability of the field current.

In a second aspect, the rotor includes a permanent magnet arranged between the claw-like magnetic pole parts adjacent to each other in a circumferential direction with an axis of easy magnetization directed in the circumferential direction of the rotor and having magnetic poles formed so that their polarities match polarities that are alternately induced in in the claw-like magnetic pole parts due to the magnetomotive force of the field winding.

According to the second aspect, the d-axis magnetic circuit and at least part of the magnet magnetic circuit through which the magnetic flux formed by the magnetic force of the permanent magnet are shared. In the shared portion of the magnet magnetic circuit and the d-axis magnetic circuit, the magnet magnetic flux flowing through the magnet magnetic circuit flows in a direction opposite to that of the magnetic flux flowing through the d-axis magnetic circuit. Therefore, the above shared portion has a large magnetic resistance and the magnetic flux hardly flows therethrough. This makes it possible to reduce the inductance of the field winding and to increase the magnetic fluxes crossing the armature winding among the magnetic fluxes formed by the permanent magnet. As a result, it is possible to increase composite magnetic fluxes that are composite magnetic fluxes of the magnetic fluxes formed by the magnetomotive force of the field winding and the magnetic fluxes formed by the permanent magnet and cross the armature winding and to increase the torque of the rotary electrical machine.

In a third aspect, the amount of ampere-turns of the field winding at which magnetic saturation of the field core occurs is made lower than the amount of ampere-turns of the armature winding at which magnetic saturation of the stator core occurs.

In the third aspect, reduction of the time constant of the electrical circuit including the field winding due to the sharp decrease of the inductance of the field winding is noticeable. With the configuration in which the reduction of the time constant is noticeable, the merit of including the switch and the controller and of setting the predetermined value as described above is large.

In a fourth aspect, a saturated magnetic flux quantity of the field core is made smaller than a saturated magnetic flux quantity of the stator core.

In the fourth aspect, the magnetomotive force necessary for saturation of the inductance of the field winding is inevitably lower than the magnetomotive force of the armature winding during torque generation of the rotary electrical machine. As a result, reduction of the time constant of the electrical circuit including the field winding due to the sharp decrease of the inductance of the field winding is noticeable. With the configuration in which the reduction of the time constant is noticeable, controllability of the field current is apt to decrease significantly, and thus the merit of including the switch and the controller and of setting the predetermined value as described above is large.

In a fifth aspect, the amount of ampere-turns of the field winding at which magnetic saturation of the field core occurs is made lower than the amount of ampere-turns of the armature winding at which magnetic saturation of the stator core occurs.

According to the fifth aspect, an increase in magnetic fluxes produced by the permanent magnet can be brought about more effectively.

In a sixth aspect, a saturated magnetic flux quantity of the field core is made smaller than a saturated magnetic flux quantity of the stator core.

According to the sixth aspect, an increase in magnetic fluxes produced by the permanent magnet can be brought about more effectively.

In a seventh aspect, a surface area of a surface opposite to the stator in the rotor is made larger than a value obtained by dividing the sectional area of the boss part when the boss part as seen in its axial direction by the number of pole pairs of the rotary electrical machine.

According to the seventh aspect, the permeance from the magnetic pole of the rotor to the stator can be increased, and a diamagnetic field acting on the permanent magnet can be reduced. For this reason, an increase in magnetic fluxes produced by the permanent magnet can be brought about more effectively.

In an eighth aspect, a magnetic path sectional area of one part of the field core is made smaller than a magnetic path sectional area of the other part, so that the permeance of the q-axis magnetic circuit is made larger than the permeance of the d-axis magnetic circuit.

According to the eighth aspect, the magnetic saturation is easily caused at one part of the field core having smaller magnetic path sectional area. As a result, the magnetic flux of the permanent magnet easily crosses the armature winding, and the permeance of the q-axis magnetic circuit can be made larger than the permeance of the d-axis magnetic circuit. On this occasion, since the permeance design can be realized by the shape of the field core, the design and processing of the field core can be easily realized.

Note that, the first aspect can be embodied, for example, as in a ninth aspect. In the ninth aspect, the controller sets the predetermined value larger as the d-axis current flowing in the armature winding is larger.

In addition, the ninth aspect can be embodied, for example, as in a tenth aspect. In the tenth aspect, the controller, based on the number of turns of the field winding, the d-axis current flowing in the armature winding, the number of turns of the armature winding, the magnetic resistance of the rotor in the d-axis magnetic circuit, and the saturated magnetic flux quantity of the rotor in the d-axis magnetic circuit, calculates an upper limit value of the field current, and sets the predetermined value to the duty ratio corresponding to the calculated upper limit value of the field current.

In an eleventh aspect, the controller, when the set duty ratio has reached the predetermined value, raises the duty ratio to 100%.

After the field current has reached its upper limit value, even if the field current is increased further, the inductance of the field winding does not change so much. For this reason, in a case where it is desired to prioritize rise of the torque of the rotary electrical machine, for example, if the resistance of the switch to overheating is large, when the field current has reached the upper limit value, it is desirable to raise the duty ratio to 100%. Then, in the eleventh aspect, the controller, when the set duty ratio has reached the predetermined value, raises the duty ratio to 100%. Since the duty ratio is set to 100%, the switch is not turned on/off, and the ripple of the field current due to on/off of the switch does not occur. Therefore, according to the eleventh aspect, it is possible to prioritize rise of the torque while suppressing ripple of the field current.

Note that, in a twelfth aspect, the field core includes a cylindrical boss part provided closer to an inner circumference side of the rotor than the field winding; a plurality of disk parts extending outward in a radial direction of the boss part from an axial one end of the boss part and provided at a predetermined angular interval in the circumferential direction of the boss part; and a plurality of claw-like magnetic pole parts extending in an axial direction of the boss part so as to surround the field winding from tips of the disk parts and having magnetic poles with alternately different polarities formed in the circumferential direction of the rotor, and when a value obtained by dividing a sectional area of the boss part when the boss part as seen in its axial direction by the number of pole pairs of the rotary electrical machine is Ab, a sectional area of the disk part is Ad, a sectional area of an annular yoke constituting the stator core is Acb, and a sectional area of a tooth per magnetic pole of the plurality of teeth constituting the stator core is At, a smaller one of Ab and Ad is made larger than a smaller one of Acb and At.

According to the twelfth aspect, the permeance of the q-axis magnetic circuit can be made larger than the permeance of the d-axis magnetic circuit.

First Embodiment

Hereinafter, a first embodiment embodying a rotary electrical machine according to the present disclosure will be described with reference to drawings. The rotary electrical machine of the present embodiment is used as an on-vehicle power generator.

As shown in FIG. 1 to FIG. 9, a rotary electrical machine 10 includes a housing 20, a stator 30, a rotor 40, a field supply part 50, and a rectifier 60. The housing 20 includes a front housing 21 and a rear housing 22. Each housing 21, 22 has a bottomed cylindrical shape with one end opened. The front housing 21 and the rear housing 22 are fastened by a fastening member 23 such as a bolt with openings abutting on each other.

The stator 30 includes an annular stator core 31 and an armature winding 32. The stator 30 is fixed to inner circumferential wall surfaces of the front housing 21 and the rear housing 22. The stator core 31 has, as shown in FIG. 4, an annular yoke 33 and a plurality of teeth 34 which protrude radially inward from the yoke 33 and are arranged at a predetermined pitch in a circumferential direction, and a slot 35 is formed between the adjacent teeth 34. Each of the teeth 34 is provided at equal intervals in the circumferential direction. Each slot 35 has an opening shape extending with the radial direction of the stator core 31 as the longitudinal direction. In the present embodiment, 96 slots 35 are formed at equal intervals in the circumferential direction of the stator core 31. The armature winding 32 has a three-phase winding and is wound around the slot 35.

The rotor 40 includes a rotary shaft 41, a field core 42, a field winding 43, and a plurality of permanent magnets 44. The rotary shaft 41 is rotatably supported via a pair of bearings 24 provided on the housing 20. The field core 42 is a Lundell-type core having first and second pole cores 42 a, 42 b fixed to an outer periphery of the rotary shaft 41. The rotor 40 is rotatably provided on the inner circumferential side of the stator 30. The rotor 40 is rotationally driven by an engine (not shown) mounted on a vehicle via a pulley 45 fixed to a front end portion of the rotary shaft 41.

The first pole core 42 a is fixed to a front end side of the rotary shaft 41, and the second pole core 42 b is fixed to a rear end side of the rotary shaft 41. The first pole core 42 a is a soft magnetic body and includes a first boss part 421 a, a first disk part 422 a, and a first claw-like magnetic pole part 423 a. In the present embodiment, the first boss part 421 a, the first disk part 422 a, and the first claw-like magnetic pole part 423 a are integrally molded to form the first pole core 42 a. The first boss part 421 a is cylindrical. The first boss part 421 a has a function of flowing a field magnetic flux on an inside of the field winding 43 in the radial direction in its axial direction. The first disk part 422 a extends radially outward from an axial one end of the first boss part 421 a and has a function of flowing a field magnetic flux in the radial direction. The first disk part 422 a is provided in plural numbers at a predetermined pitch in the circumferential direction. In the present embodiment, eight first disk parts 422 a are provided at equal interval in the circumferential direction. The first claw-like magnetic pole part 423 a extends in the axial direction so as to surround the field winding 43 from the tip of the first disk part 422 a on the outer circumferential side of the first boss part 421 a and has a function of exchanging magnetic fluxes with the stator core 31. The first claw-like magnetic pole part 423 a is provided corresponding to the first disk part 422 a and specifically eight ones are provided. The first claw-like magnetic pole part 423 a has a trapezoidal shape with its root side as a long side and its tip side as a short side, and the sectional area becomes smaller as it goes from the root side toward the tip side.

The second pole core 42 b is a soft magnetic body and includes a second boss part 421 b, a second disk part 422 b, and a second claw-like magnetic pole part 423 b. In the present embodiment, the shape of the second pole core 42 b is the same as the shape of the first pole core 42 a. Therefore, a detailed description of the second boss part 421 b, the second disk part 422 b, and the second claw-like magnetic pole part 423 b will be omitted.

The first pole core 42 a and the second pole core 42 b are in a state in which the first claw-like magnetic pole parts 423 a and the second claw-like magnetic pole parts 423 b are made to face each other alternately, and an axial rear end surface of the first pole core 42 a and an axial front end surface of the second pole core 42 b are abutted to each other. Thereby, the first claw-like magnetic pole parts 423 a and the second claw-like magnetic pole parts 423 b are arranged alternately in the circumferential direction. Therefore, in the present embodiment, each pole core 42 a, 42 b has eight N poles and eight S poles and constitutes a Lundell-type rotor core having 16 poles.

The field winding 43 is wound on the outer circumferential side of the first and second boss parts 421 a and 421 b in a state of being isolated from the field core 42 and is surrounded by the first and second claw-like magnetic pole parts 423 a and 423 b.

As shown in FIG. 2 to FIG. 5, the rotor 40 includes the permanent magnet 44 arranged between the first claw-like magnetic pole part 423 a and the second claw-like magnetic pole part 423 b adjacent to each other in the circumferential direction. In the present embodiment, 16 permanent magnets 44 are provided. The permanent magnet 44 has a rectangular shape and its magnetization-easy axis is directed in the circumferential direction of the rotor 40. In a state in which the magnetic pole of one end of the permanent magnet 44 is abutted to the first claw-like magnetic pole part 423 a and the magnetic pole of the other end of the permanent magnet 44 is abutted to the second claw-like magnetic pole part 423 b, the permanent magnet 44 is held by each claw-like magnetic pole part 423 a, 423 b. When the field current flows through the field winding 43, a magnetomotive force is generated in each boss part 421 a, 421 b. Thereby, magnetic poles having different polarities are formed in the first and second claw-like magnetic pole parts 423 a and 423 b, respectively. That is, of the N and S magnetic poles, the first claw-like magnetic pole part 423 a is magnetized to one polarity and the second claw-like magnetic pole part 423 b is magnetized to the other polarity. In this case, in the permanent magnet 44, the magnetic poles are formed so as to match polarities are alternately induced ining in the first and second claw-like magnetic pole parts 423 a and 423 b by the magnetomotive force of the field winding 43.

As shown in FIG. 1 and FIG. 9, the field supply part 50 includes one pair of slip rings 51, one pair of brushes 52, a regulator 53, and a capacitor 54. Each slip ring 51 is provided on the axial rear end side of the rotary shaft 41. Each brush 52 is provided with its tip pressed to the surface of the slip ring 51. The brush 52 feeds power to the field winding 43 via the slip ring 51.

The regulator 53 is a device that regulates an output voltage of the rotary electrical machine 10 by controlling a field current flowing in the field winding 43. The regulator 53 includes a switching element 53 a and a reflux diode 53 b. In the present embodiment, the switching element 53 a is connected in series with the field winding 43 and is a MOSFET. The reflux diode 53 b is connected in parallel with the field winding 43. The capacitor 54 is connected in parallel with a serial connection body of the switching element 53 a and the reflux diode 53 b. Specifically, the cathode of the reflux diode 53 b is connected to the first end of the capacitor 54, and the source of the switching element 53 a is connected to the second end of the capacitor 54. When the switching element 53 a is turned on, a closed circuit including the field winding 43 and the capacitor 54 is formed, and the electric power is supplied from the power source to the field winding 43. On the other hand, when the switching element 53 a is turned off, the closed circuit is not formed, and a magnetic energy accumulated in the field winding 43 is released as a current through the reflux diode 53 b.

The rectifier 60 is electrically connected to the armature winding 32 and is a device that rectifies an AC current output from the armature winding 32 into a DC current. In the present embodiment, the rectifier 60 is composed of a plurality of diodes which are rectifier elements.

The rotary electrical machine 10 includes a field current detector 70 for detecting a field current Ifr flowing through the field winding 43. In the present embodiment, the field current detector 70 is provided on the source side of the switching element 53 a. The detection value of the field current detector 70 is input to a controller 55 included in the field supply part 50. The controller 55 turns on/off the switching element 53 a. The controller 55 receives a phase current that is detected by a phase current detector and flows through the armature winding 32 and an electric angle θe of the rotary electrical machine 10 detected by an angle detector. Note that, the phase current detector and the angle detector may be included in the rotary electrical machine 10 or may be included in a system outside the rotary electrical machine 10.

In the rotary electrical machine 10 having the above configuration, when a rotational force is transmitted to the pulley 45 via a belt or the like, the rotor 40 rotates in a predetermined direction together with the rotary shaft 41. When an exciting voltage is applied from the brush 52 to the field winding 43 via the slip ring 51 in this state, the first and second claw-like magnetic pole parts 423 a and 423 b are excited, and the N magnetic poles and S magnetic poles are alternately formed in the circumferential direction of the rotor 40. Thereby, a rotating magnetic field is applied to the armature winding 32, and an AC current flows from the armature winding 32 to the rectifier 60. The AC current is converted into a DC current by the rectifier 60. The converted DC current is supplied to the field winding 43 as a power generation current Ig or supplied to an external power supply target 80 via an output terminal TB of the rotary electrical machine 10. The power supply target 80 includes a battery.

Subsequently, a magnetic circuit of the rotary electrical machine 10 will be described with reference to FIGS. 4, 5, and 8.

When the field current flows through the field winding 43, a field magnetic flux passing through the first and second boss parts 421 a and 421 b and one pair of the first and second claw-like magnetic pole parts 423 a and 423 b is formed. A d-axis magnetic circuit 81 is formed by this field magnetic flux. The d-axis magnetic circuit 81 is, as indicated with an arrow of a broken line in FIG. 4, a magnetic circuit in which the magnetic flux enters the first claw-like magnetic pole part 423 a from the d-axis teeth 34 of the stator core 31 and returns to the d-axis teeth 34 at a position shifted by one magnetic pole of the stator core 31 via the first disk part 422 a, the first boss part 421 a, the second boss part 421 b, the second disk part 422 b, and the second claw-like magnetic pole part 423 b, and then returns again to the d-axis teeth 34 at a position shifted by one magnetic pole through the yoke 33. The d-axis magnetic circuit 81 is a magnetic circuit that generates an inverse electromotive force in the rotor 40.

As shown in FIG. 6, a value obtained by dividing the sectional area of the boss part 421 a, 421 b when the boss part 421 a, 421 b is seen in the axial direction by the number of pole pairs Pn (Pn=8) of the rotary electrical machine 10 is Ab, and the sectional area of the disk part 422 a, 422 b is Ad. As shown in FIG. 4, the sectional area of the yoke 33 is Acb, and the sectional area of the teeth 34 per magnetic pole is At. The teeth 34 per magnetic pole are teeth 34 facing one claw-like magnetic pole part, and in the present embodiment, the number of teeth 34 per magnetic pole is three. In this case, as shown in FIG. 7, when the smaller one of Ab and Ad is Art, and the smaller one of Acb and At is Ast, Art>Ast is established. That is, in the d-axis magnetic circuit 81, the magnetic path sectional area in the magnetic circuit on the rotor 40 side is made smaller than the magnetic path sectional area in the magnetic circuit on the stator 30 side. According to this configuration, when the magnetic circuit passing through the q-axis shifted by 90° in the electric angle from the d-axis is a q-axis magnetic circuit 82 (refer to FIG. 4), a permeance Pst of the q-axis magnetic circuit 82 can be made larger than a permeance Prt of the d-axis magnetic circuit 81.

The permanent magnet 44 arranged between the first and second claw-like magnetic pole parts 423 a and 423 b adjacent to each other in the circumferential direction forms, as shown in FIG. 8, a first magnet magnetic circuit 83 and a second magnet magnetic circuit 84. The first magnet magnetic circuit 83 is a magnetic circuit through which of the magnet magnetic fluxes a magnetic flux crossing the stator 30 flows. The second magnet magnetic circuit 84 is a magnetic circuit in which of the magnet magnetic fluxes a magnetic flux passes through the boss part 421 a, 421 b and the disk part 422 a, 422 b and flow of the magnetic flux completes in the rotor 40.

The first magnet magnetic circuit 83 and the d-axis magnetic circuit 81 share the magnetic circuit that starts from the second claw-like magnetic pole part 423 b and returns to the first claw-like magnetic pole part 423 a via the stator 30. In addition, the second magnet magnetic circuit 84 and the d-axis magnetic circuit 81 share the magnetic circuit in the boss parts 421 a and 421 b and the disk parts 422 a and 422 b. Since the magnet magnetic flux flowing through the second magnet magnetic circuit 84 flows in a direction opposite to that of the magnetic flux flowing through the d-axis magnetic circuit 81, the magnetic resistance is large, and the magnetic flux flow is impeded. Thereby, of the first and second magnet magnetic circuits 83 and 84, a magnet magnetic flux of the magnet magnetic circuit through which the magnetic flux crossing the stator 30 flows increases. As a result, the magnet magnetic flux can be used effectively, and electric power generation by the rotary electrical machine 10 can be substantially improved.

In the present embodiment, a surface area As of the surface opposite to the stator 30 in the rotor 40 is made larger than a value Ab obtained by dividing the sectional area of the boss part 421 a, 421 b when the boss part 421 a, 421 b as seen in its axial direction by the number of pole pairs. Thereby, the permeance from the magnetic pole on the rotor 40 side to the stator 30 side can be increased, and a diamagnetic field acting on the permanent magnet 44 can be reduced. For this reason, an increase in magnetic fluxes produced by the permanent magnet 44 can be brought about more effectively, which contributes to improvement of electric power generation by the rotary electrical machine 10.

Note that, in the present embodiment, the surface area As is defined as a surface area of the outer circumferential surface of the claw-like magnetic pole part 423 a, 423 b. As shown in FIG. 7, the width dimension of a root portion of the claw-like magnetic pole part 423 a, 423 b in the circumferential direction of the rotor 40 or the disk part 422 a, 422 b is Wrr, and the width dimension of the tip portion of the claw-like magnetic pole part 423 a, 423 b in the circumferential direction of the rotor 40 is Wte. In addition, the height dimension of the claw-like magnetic pole part 423 a, 423 b in the axial direction is Ht. As shown in FIG. 6, a surface facing the stator 30 in the radial direction in the disk part 422 a, 422 b is referred to as a disk guide. The length dimension of the disk guide in the axial direction of the rotor 40 is Hdg. In this case, the surface area As is calculated by “As=(Wte+Wrr)×Ht/2+Hdg×Wrr”. Note that, in the present embodiment, the width dimension Wrr, Wte in the circumferential direction is measured by a direct distance without taking a curvature into consideration. In addition, a notch portion, an R portion, and a chamfered portion provided at the disk part 422 a, 422 b, the claw-like magnetic pole part 423 a, 423 b, and the stator core 31 for the purpose of magnet insertion or reinforcement do not greatly influence the calculation of the surface area As.

In the present embodiment, the amount of ampere-turns Ir of the field winding 43 at which magnetic saturation of the field core 42 occurs is made lower than the amount of ampere-turns Is of the armature winding 32 at which magnetic saturation of the stator core 31 occurs. Thereby, an increase in magnetic fluxes produced by the permanent magnet 44 can be brought about more properly. In addition, a saturated magnetic flux quantity Φr of the field core 42 is made smaller than a saturated magnetic flux quantity Φs of the stator core 31. Thereby, the increase in magnetic fluxes produced by the permanent magnet 44 can be brought about more.

Incidentally, in the rotary electrical machine 10 in which the permeance Pst of the q-axis magnetic circuit 82 is made larger than the permeance Prt of the d-axis magnetic circuit 81, as shown in FIG. 10, when the field current is increased, the inductance of the field winding 43 sharply decreases before the torque of the rotary electrical machine 10 reaches its maximum value. Specifically, the inductance decreases at a level that changes by an order of magnitude. Generally, when a permeability of a material of the magnetic circuit is μ, a magnetic path sectional area is A, and a magnetic path length is sL, a permeance P of the magnetic circuit is expressed by “P=μ×A/sL”. In addition, when the number of turns of the field winding 43 is Nf, an inductance Lrt of the field winding 43 is expressed by “Lrt=Prt×Nf{circumflex over ( )}2”. The permeability μ changes according to the degree of magnetic saturation of the magnetic circuit, and its magnitude is about several thousands to 10,000 times the permeability of air in a state before saturation. However, in a state of oversaturation in which saturation has further advanced, it decreases to about several times the permeability of air. When the inductance of the field winding 43 is Lrt, and a resistance of the field winding 43 is R, a time constant τ of a general electric circuit including the field winding 43 is expressed by Lrt/R. In the present embodiment, although the resistance R of the field winding 43 does not change greatly according to a load state of the rotary electrical machine 10, when the inductance Lrt changes at a level that changes by an order of magnitude, the time constant τ sharply changes. As a result, the ripple of the field current is increased, and the control of the field current becomes unstable. This may cause large fluctuation in the generated voltage output from the output terminal TB of the rotary electrical machine 10.

Thus, in the present embodiment, the controller 55 performs processing shown in FIG. 11. FIG. 11 shows a block diagram of control processing of the field current.

A d-axis current calculating part 55 a, based on the detected phase current and electric angle θe, calculates the d-axis current Id flowing in the armature winding 32.

A limiter 55 b limits a field command current Iftgt input from the outside by a field threshold Ifth. Specifically, when the input field command current Iftgt is equal to or less than the field threshold Ifth, the limiter 55 b outputs the input field command current Iftgt as it is. On the other hand, when the input field command current Iftgt exceeds the field threshold Ifth, the limiter 55 b outputs the field command current Iftgt having the same value as the field threshold Ifth. Note that, the field command current Iftgt is set large, for example, when it is desired to increase a charge current of a battery included in the power supply target 80.

A deviation calculating part 55 c calculates a current deviation ΔIf by subtracting the field current Ifr detected by the field current detector 70 from the field command current Iftgt output from the limiter 55 b.

A duty calculating part 55 d, based on the current deviation ΔIf, calculates a duty ratio Duty that is an operation amount for feedback-controlling the field current Ifr to the field command current Iftgt. The duty ratio Duty is a ratio of an on-time Ton to one switching cycle Tsw of the switching element 53 a. The switching element 53 a is operated based on the duty ratio Duty calculated by the duty calculating part 55 d.

The limiter 55 b calculates the field threshold Ifth based on the d-axis current Id calculated by the d-axis current calculating part 55 a and the following equation (eq1). In the following equation (eq1), Ns indicates the number of turns of the armature winding 32, Rd indicates the magnetic resistance of the rotor 40 in the d-axis magnetic circuit 81, and Φ indicates the saturated magnetic flux quantity of the rotor 40 in the d-axis magnetic circuit 81. Note that, the saturated magnetic flux quantity Φ can be calculated by “Φ=Bs×Ad” where Ad is a magnetic path sectional area of the rotor 40 in the d-axis magnetic circuit 81, and Bs is a saturated magnetic flux density of an iron core material of the rotor 40. Ifth=(Ns×Id+Rd×Φ)/Nf  (eq1)

The field threshold Ifth, as shown in FIG. 12, becomes larger as the d-axis current Id becomes larger. The field threshold Ifth, as shown in FIG. 10, is a value larger than a field current Ifa that gives the maximum reduction amount of the inductance of the field winding 43 with respect to an increasing amount of the field current in a range that the field current can take during driving the rotary electrical machine 10. The field threshold Ifth is a field current immediately before the inductance of the field winding 43 saturates. In the present embodiment, the saturation of the inductance means a state in which the inductance does not substantially change even if the field current increases. Since the field command current Iftgt is limited by the field threshold Ifth in the limiter 55 b, the duty ratio Duty calculated by the duty calculating part 55 d is set to a value that is larger than the duty ratio Duty corresponding to the maximum field current Ifa and less than 100%. Note that, the duty ratio Duty corresponding to the field threshold Ifth corresponds to the predetermined value.

According to the processing of FIG. 11 including the limiter 55 b, as shown in FIG. 13, the ripple of the field current Ifr can be suppressed, and reduction in controllability of the field current Ifr can be suppressed. As a result, the generated voltage of the rotary electrical machine 10 can be stabilized. In contrast, in a comparative example in which the field current Ifr becomes larger than the field threshold Ifth, as shown in FIG. 14, the ripple of the field current Ifr increases, and the controllability of the field current Ifr is greatly lowered. Note that, FIG. 13 and FIG. 14 show changes of the field current Ifr during starting the rotary electrical machine 10 in which the inductance of the field winding 43 changes greatly and the controllability of the field current Ifr is easily lowered.

According to the present embodiment described above, it is possible to appropriately suppress reduction in the controllability of the field current and to stabilize the generated voltage of the rotary electrical machine 10.

Second Embodiment

Hereinafter, the second embodiment will be described with reference to drawings centering on the difference from the first embodiment. In the present embodiment, as shown in FIG. 15, the configuration of the field supply part 50 is changed. Note that, in FIG. 15, the same configuration as that shown in previous FIG. 9 is attached with the same code for convenience sake.

A rectifier 61 is configured as an inverter including upper and lower arm switching elements SWH, SWL for three phases and performs synchronous rectification. The synchronous rectification can reduce thermal loss. In the present embodiment, each switching element SWH, SWL is a MOSFET.

The field supply part 50 includes first and second switching elements 56 a and 56 b. In the present embodiment, each of the first and second switching elements 56 a and 56 b is a MOSFET. The drain of the first switching element 56 a is connected to the drain of the upper arm switching element SWH of the rectifier 61, and the drain of the second switching element 56 b is connected to the source of the first switching element 56 a. The source of the lower arm switching element SWL is connected to the source of the second switching element 56 b. The capacitor 54 is connected in parallel to a serial connection body of the first and second switching elements 56 a and 56 b.

To the drain of the first switching element 56 a, the first end of the field winding 43 is connected via the brush 52 and the slip ring 51. To the second end of the field winding 43, the connection point of the first and second switching elements 56 a and 56 b is connected via the slip ring 51 and the brush 52.

The controller 55 turns on/off the upper and lower arm switching elements SWH and SWL in order to perform synchronous rectification. In addition, the controller 55 turns on/off the first and second switching elements 56 a and 56 b. When the first switching element 56 a is turned off and the second switching element 56 b is turned on, a closed circuit including the capacitor 54, the field winding 43, and the second switching element 56 b is formed, and electric power is supplied from the capacitor 54 to the field winding 43. On the other hand, when the first switching element 56 a is turned on and the second switching element 56 b is turned off, a closed circuit including the capacitor 54, the field winding 43, and the second switching element 56 b is not formed, and electric power is not supplied from the capacitor 54 to the field winding 43. Note that, in the present embodiment, the duty ratio Duty is a ratio of the on-time Ton to the first switching cycle Tsw of the second switching element 56 b.

Note that, the switching frequency of the first and second switching elements 56 a and 56 b is made smaller than the switching frequency of the switching elements SWH and SWL forming the rectifier 61. This makes it possible to suppress electromagnetic interference to the rectifier 61 due to the switching operation of the first and second switching elements 56 a and 56 b and to reduce the switching loss of the first and second switching elements 56 a and 56 b. By reducing the switching loss, it is possible to reduce thermal burden from the first and second switching elements 56 a and 56 b to the rectifier 61.

Other Embodiments

Note that, the above-described each embodiment may be modified as described below and carried out.

-   -   Since the magnetic path sectional area of one part of the field         core 42 is made smaller than the magnetic path sectional area of         the other part, the permeance Pst of the q-axis magnetic circuit         82 may be made larger than the permeance Prt of the d-axis         magnetic circuit 81. In this case, since the permeance design         can be realized by the shape of the field core 42, the design         and processing of the field core 42 can be easily realized. In         the following, the configuration in which the magnetic path         sectional area of one part of the field core 42 is made smaller         than the magnetic path sectional area of the other part will be         described with the first pole core 42 a as an example.

As shown in FIG. 16, an annular groove portion 422 c extending in the circumferential direction of the rotor 40 may be formed in the first disk part 422 a. In this case, in the first disk part 422 a, a sectional area Ad of a portion where the groove portion 422 c is not formed is larger than a sectional area A0 of a portion where the groove portion 422 c is formed.

As shown in FIG. 17, a central recessed portion 422 d extending in a radial direction of the rotor 40 may be formed in the first disk part 422 a, and a recessed portion 422 e extending in the circumferential direction may be formed between the first disk parts 422 a adjacent to each other in the circumferential direction of the rotor 40.

As shown in FIG. 18, a groove portion 421 c may be formed in the first boss part 421 a along its circumferential direction.

As shown in FIG. 19, a part 421 d in which the outer diameter on the tip side of the first boss part 421 a is made smaller than the outer diameter on the root side may be formed.

As shown in FIG. 20, a taper part 421 e communicating with a hole part 421 f into which the rotary shaft 41 is inserted may be formed on the first disk part 422 a side in the first boss part 421 a. In this case, the first pole core 42 a may be composed of two members. FIG. 20 shows a boundary M at which two members contact with each other.

As shown in FIG. 21, a taper part 421 g communicating with the hole part 421 f into which the rotary shaft 41 is inserted may be formed on the opposite side of the first disk part 422 a in the first boss part 421 a.

As shown in FIG. 22, an annular groove portion 421 h extending in the circumferential direction may be formed at an end portion on the opposite side of the first disk part 422 a in the first boss part 421 a. Note that, a plurality of the groove portions may be formed as shown in FIG. 23. FIG. 23 shows two annular groove portions 421 j and 421 k.

For example, by using the configuration shown in FIGS. 16 to 23, it is possible to change the permeance locally in a portion other than a claw-like magnetic pole part where the permeance is desired to be maintained high in the field core, while maintaining a high saturated magnetic flux density, that is, a high magnetic permeability in the claw-like magnetic pole part. Therefore, influence on the performance of the rotary electrical machine 10 can be reduced.

-   -   When the duty ratio Duty set in the duty calculating part 55 d         has reached the duty ratio Duty corresponding to the field         threshold Ifth, processing of raising the duty ratio Duty to         100% may be performed. FIG. 24 shows this processing procedure.         The processing shown in FIG. 24 is executed by the controller         55, for example, at each predetermined processing cycle.

In step S10, a field command current Iftgt is obtained. In subsequent step S11, it is determined whether the obtained field command current Iftgt has reached the field threshold Ifth. When the negative determination is made in step S11, the process proceeds to step S12, where the duty ratio Duty is calculated with the method shown in FIG. 11. On the other hand, when the affirmative determination is made in step S11, the process proceeds to step S13, where the duty ratio Duty is raised to 100%, for example, in a stepwise manner.

After the field current Ifr has reached the field threshold Ifth, even if the field current Ifr is increased furthermore, the inductance of the field winding 43 does not change so much as shown in FIG. 10. For this reason, in a case where high priority is desired to be given to start-up of electric power generation by the rotary electrical machine 10, when the field current Ifr has reached the field threshold Ifth, if resistance to overheat of the switching element 53 a is large, it is desirable to raise the duty ratio Duty to 100% at once. Then, processing shown in FIG. 24 is performed. Since the duty ratio Duty is set to 100%, the switching element 53 a is not turned on/off, and the ripple of the field current due to on/off switching of the switching element 53 a does not occur. Therefore, it is possible to give priority to start-up of electric power generation by the rotary electrical machine 10 while suppressing the ripple of the field current.

-   -   The permanent magnet 44 may not be provided in the rotor 40. In         this case, reduction of the inductance of the field winding 43         relative to increase of the field current may be noticeable.

Note that, when the permanent magnet 44 is not provided, since the amount of ampere-turns Ir of the field winding 43 at which magnetic saturation of the field core 42 occurs is made lower than the amount of ampere-turns Is of the armature winding 32 at which magnetic saturation of the stator core 31 occurs, reduction of the time constant on the side of the rotor 40 including the field winding 43 is noticeable. In addition, when the permanent magnet 44 is not provided, since the saturated magnetic flux quantity Φr of the field core 42 is made smaller than the saturated magnetic flux quantity Φs of the stator core 31, the above reduction of the time constant is noticeable. In this case, the merit of including the limiter 55 b shown in FIG. 11 is large.

-   -   As the rotary electrical machine, not only the one used only as         a power generator, but also, for example, the one used as a         power generator or an electric motor, such as ISG (integrated         Starter Generator), or the one used only as an electric motor         may be used. In addition, as the rotary electrical machine, it         is not limited to the one mounted on a vehicle.

The present disclosure is described according to the embodiments, but it should be appreciated that the present disclosure is not limited to the embodiments and structures. The present disclosure also includes various variations and modifications within an equivalent range. In addition, various combinations and forms, and further other combinations and forms including only one element, more than that or less than that in addition to the various combinations and forms are also included in a category and concept of the present disclosure. 

What is claimed is:
 1. A rotary electrical machine comprising: an annular stator having a stator core around which an armature winding is wound, and a rotor arranged on an inner circumference side of the stator, wherein the rotor includes: a field core including a cylindrical boss part and a plurality of claw-like magnetic pole parts arranged on an outer circumferential side of the boss part and having magnetic poles with alternately different polarities formed in a circumferential direction of the rotor, and a field winding that is wound on the outer circumferential side of the boss part and generates a magnetomotive force by supplying a field current, wherein in a case where a magnetic circuit in which a magnetic flux formed by a magnetomotive force of the field winding flows and which passes through a d-axis via the boss part, a pair of the claw-like magnetic pole parts and the stator core is referred to as a d-axis magnetic circuit, and a magnetic circuit which is formed by a current flowing through the armature winding and which passes through a q-axis shifted by 90° in an electric angle from the d-axis is referred to as a q-axis magnetic circuit, a permeance of the q-axis magnetic circuit is made larger than a permeance of the d-axis magnetic circuit, wherein the rotary electrical machine further comprising: a switch provided so as to supply power from a power source to the field winding by being turned on and stop supplying power from the power source to the field winding by being turned off; and a controller that, in a case where a ratio of an on-time to one switching cycle of the switch is defined as a duty ratio, calculates the duty ratio on the condition that an upper limit of the duty ratio is set as a predetermined value and turns on/off the switch based on the calculated duty ratio, wherein the predetermined value is set to a value which is larger than the duty ratio corresponding to the field current that gives the maximum reduction amount of the inductance of the field winding with respect to an increasing amount of the field current in a range that the field current can take and which is less than 100%.
 2. The rotary electrical machine according to claim 1, wherein the rotor includes a permanent magnet arranged between the claw-like magnetic pole parts adjacent to each other in a circumferential direction with an axis of easy magnetization directed in the circumferential direction of the rotor and having magnetic poles formed so that their polarities match polarities that are alternately induced in in the claw-like magnetic pole parts due to the magnetomotive force of the field winding.
 3. The rotary electrical machine according to claim 1, wherein an the amount of ampere-turns of the field winding at which magnetic saturation of the field core occurs is made lower than an the amount of ampere-turns of the armature winding at which magnetic saturation of the stator core occurs.
 4. The rotary electrical machine according to claim 1, wherein a saturated magnetic flux quantity of the field core is made smaller than a saturated magnetic flux quantity of the stator core.
 5. The rotary electrical machine according to claim 2, wherein an the amount of ampere-turns of the field winding at which magnetic saturation of the field core occurs is made lower than an the amount of ampere-turns of the armature winding at which magnetic saturation of the stator core occurs.
 6. The rotary electrical machine according to claim 2, wherein a saturated magnetic flux quantity of the field core is made smaller than a saturated magnetic flux quantity of the stator core.
 7. The rotary electrical machine according to claim 2, wherein a surface area of a surface opposite to the stator in the rotor is made larger than a value obtained by dividing the sectional area of the boss part when the boss part as seen in its axial direction by the number of pole pairs of the rotary electrical machine.
 8. The rotary electrical machine according to claim 1, wherein in a case where a magnetic circuit in which a magnetic flux formed by a magnetomotive force of the field winding flows and which passes through a d-axis via the boss part, a pair of the claw-like magnetic pole parts and the stator core is referred to as a d-axis magnetic circuit, and a magnetic circuit which is formed by a current flowing through the armature winding and which passes through a q-axis shifted by 90° in an electric angle from the d-axis is referred to as a q-axis magnetic circuit, since one part of a magnetic path sectional area of the field core is made smaller than the other part of the magnetic path sectional area, a permeance of the q-axis magnetic circuit is made larger than a permeance of the d-axis magnetic circuit.
 9. The rotary electrical machine according to claim 1, wherein the controller sets the predetermined value larger as the d-axis current flowing in the armature winding is larger.
 10. The rotary electrical machine according to claim 9, wherein in a case where a magnetic circuit in which a magnetic flux formed by a magnetomotive force of the field winding flows and which passes through a d-axis via the boss part, a pair of the claw-like magnetic pole parts and the stator core is referred to as a d-axis magnetic circuit, the controller, based on the number of turns of the field winding, the d-axis current flowing in the armature winding, the number of turns of the armature winding, the magnetic resistance of the rotor in the d-axis magnetic circuit, and the saturated magnetic flux quantity of the rotor, calculates an upper limit value of the field current, and sets the predetermined value to the duty ratio corresponding to the calculated upper limit value of the field current.
 11. The rotary electrical machine according to claim 9, wherein the controller, when the set duty ratio has reached the predetermined value, raises the duty ratio to 100%.
 12. A rotary electrical machine comprising: an annular stator having a stator core around which an armature winding is wound, and a rotor arranged on an inner circumference side of the stator, wherein the rotor includes a field core and a field winding that generates a magnetomotive force by supplying a field current, wherein the field core includes: a cylindrical boss part provided closer to an inner circumference side of the rotor than the field winding; a plurality of disk parts extending outward in a radial direction of the boss part from an axial one end of the boss part and provided at a predetermined angular interval in the circumferential direction of the boss part; and a plurality of claw-like magnetic pole parts extending in an axial direction of the boss part so as to surround the field winding from tips of the disk parts and having magnetic poles with alternately different polarities formed in the circumferential direction of the rotor, wherein when a value obtained by dividing a sectional area of the boss part when the boss part as seen in its axial direction by the number of pole pairs of the rotary electrical machine is Ab, a sectional area of the disk part is Ad, a sectional area of an annular yoke constituting the stator core is Acb, and a sectional area of a tooth per magnetic pole of the plurality of teeth constituting the stator core is At, a smaller one of Ab and Ad is made larger than a smaller one of Acb and At, wherein the rotary electrical machine further comprising: a switch provided so as to supply power from a power source to the field winding by being turned on and stop supplying power from the power source to the field winding by being turned off; and a controller that, in a case where a ratio of an on-time to one switching cycle of the switch is defined as a duty ratio, sets the duty ratio on the condition that an upper limit of the duty ratio is set as a predetermined value and turns on/off the switch based on the set duty ratio, wherein the predetermined value is set to a value which is larger than the duty ratio corresponding to the field current that gives the maximum reduction amount of the inductance of the field winding with respect to an increasing amount of the field current in a range that the field current can take and which is less than 100%.
 13. The rotary electrical machine according to claim 12, wherein the rotor includes a permanent magnet arranged between the claw-like magnetic pole parts adjacent to each other in a circumferential direction with an axis of easy magnetization directed in the circumferential direction of the rotor and having magnetic poles formed so that their polarities match polarities that are alternately induced in in the claw-like magnetic pole parts due to the magnetomotive force of the field winding.
 14. The rotary electrical machine according to claim 12, wherein an the amount of ampere-turns of the field winding at which magnetic saturation of the field core occurs is made lower than an the amount of ampere-turns of the armature winding at which magnetic saturation of the stator core occurs.
 15. The rotary electrical machine according to claim 12, wherein a saturated magnetic flux quantity of the field core is made smaller than a saturated magnetic flux quantity of the stator core.
 16. The rotary electrical machine according to claim 13, wherein an the amount of ampere-turns of the field winding at which magnetic saturation of the field core occurs is made lower than an the amount of ampere-turns of the armature winding at which magnetic saturation of the stator core occurs.
 17. The rotary electrical machine according to claim 13, wherein a saturated magnetic flux quantity of the field core is made smaller than a saturated magnetic flux quantity of the stator core.
 18. The rotary electrical machine according to claim 13, wherein a surface area of a surface opposite to the stator in the rotor is made larger than a value obtained by dividing the sectional area of the boss part when the boss part as seen in its axial direction by the number of pole pairs of the rotary electrical machine.
 19. The rotary electrical machine according to claim 12, wherein in a case where a magnetic circuit in which a magnetic flux formed by a magnetomotive force of the field winding flows and which passes through a d-axis via the boss part, a pair of the claw-like magnetic pole parts and the stator core is referred to as a d-axis magnetic circuit, and a magnetic circuit which is formed by a current flowing through the armature winding and which passes through a q-axis shifted by 90° in an electric angle from the d-axis is referred to as a q-axis magnetic circuit, one part of a magnetic path sectional area of the field core is made smaller than the other part of the magnetic path sectional area, so that a permeance of the q-axis magnetic circuit is made larger than a permeance of the d-axis magnetic circuit.
 20. The rotary electrical machine according to claim 12, wherein the controller sets the predetermined value larger as the d-axis current flowing in the armature winding is larger. 